Microelectric circuits formed by means of thick-film technology are fabricated by using a paste formulated from a material which includes a conductor, resistor or dielectric. The paste is formulated using metal powders, a glass frit powder, binders and vehicles and placed or deposited (e.g.-by screen printing) in a selected pattern onto a non-conductive substrate (e.g. ceramic, porcelainized steel, glass, etc.), and then dried to volatilize vehicle constituents contained in the paste such that it can then be fired to sinter or fuse the constituents bonding the film to the substrate.
In the past, the conductors have been made using a precious metal conductive paste prepared by using a noble metal or combination of noble metals such as platinum, palladium, silver or gold to permit electrical flow with minimal opposition.
The thick-film microelectronic circuit industry has begun to replace the precious metal conductors with copper conductors. This in turn has created a problem in that the copper conductor paste containing polymers, glass frit and copper metal together with other metal oxides must be processed in a nitrogen-based atmosphere. The use of the nitrogen-based atmosphere is dictated in order to prevent excessive oxidation of the copper. The previously used precious metal conductors could be fired in a furnace atmosphere of air because oxidation was not a problem. The lack of oxygen in the furnace atmosphere can greatly affect the behavior of the polymers and glass frit during firing which in turn affects physical and electrical characteristics of the fired conductor. Because there is insufficient oxygen present to effectively oxidize the carbon-based polymers and keep various metal oxides from reducing, pyrolyzation or volatilization of the polymeric or metallic constituents can cause sooting problems in the furnace and on the parts. Condensation of the vaporized vehicle or volatilized metals such as bismuth can occur on the cooler areas of the furnace. The sooting and condensation creates two problems for the thick-film manufacture, first, the condensed polymers that attach to the interior of the furnace walls make it necessary to clean the furnace on a regular basis to prevent deposits from dripping onto to the belt and the parts being processed, and, second, soot and condensed polymers deposit onto the surface of a copper conductor and the ceramic substrate. In order to solve the first problem, the furnace must be cooled to enable the cleaning operation to proceed thus diminishing available production time in the furnace. As to the second problem, deposits on the surface of a conductor are aesthetically undesirable but furthermore can compromise the quality of the circuit during later processing steps such as wire bonding or soldering. Thus it is necessary to clean the substrates in some manner before proceeding to package the substrate causing added expense to the manufacturer.
Due to the fact that there is insufficient oxygen present in the furnace atmosphere, glass frit contained in the copper thick-film paste does not form the necessary bonding interface between the copper metal particles and the ceramic substrate. A lack of, or incomplete formation of, this bonding interface results in less than desirable adhesion strengths between the conductor and the ceramic substrate with concomitant increased resistivity.
In attempting to eliminate the vehicle burn-off problem and achieve optimum physical and electrical properties, present day manufacturers have taken several approaches. One approach is to inject small amounts D of oxygen (e.g. up to 1% by volume, normally 5 to 15 parts per million) have been added to the preheat or burn out zone of conventional infrared and near-infrared furnaces commonly used in the manufacture of thick-film components. At the low temperature and short residence time normally found in the preheat zone of these furnaces, the oxygen is not as severely oxidizing as at higher temperatures, but can oxidize some of the carbon-based vehicles and provide a slight oxidation potential to form the desired glassy bonding interface. This technique is not completely effective possibly because of the low temperatures in which the reactants are present. Oxidation of the copper could also occur should oxygen migrate into the hotter areas of the furnace, thus ruining the conductor. An other approach resides in the manufacturers making physical modifications to the furnaces, be they conventional, infrared or near-infrared in an attempt to evacuate the reaction products from the preheat zone more effectively. Although these methods have met with limited success D in removal of vehicles from the furnace, they require the use of large volumes of gas, thus increasing production costs. Another approach has been to modify the paste formulation in an attempt to eliminate the sooting problem and provide optimal conductivity, solderability and adhesion properties to the finished part, all with little proven success to date.
A more complete discussion of processes for manufacturing thickfilm electrical components is contained in the specification of U.S. Pat. No. 4.622.240 which is incorporated herein by reference.